ok so on my '96 XJ Sport 4.0L H.O. my oil pressure gauge is showing something strange.....it used to be at 0 when the engine was off then around 40 at an idle and go up a bit when i'm on the highway.....but recently the gauge has been saying 40 even when the engine is off (and has been off for a while) and says about 60 at an idle and goes up a little from there when I drive......

is it my gauge thats messed up or is it my oil pump or filter or what? is this a big problem?

Oil temp sending units are a very common failure on the Jeep 4.0 engine. And when they go south, they often exhibit the symptoms you are describing.

Some guys temporarily put a mechanical oil pressure gauge where the sending unit resides to verify their exact oil pressure, but my bet is that a new sending unit will restore your oil pressure reading accuracy. It is located directly above the oil filter.

You might want to try cleaning the electrical connector to the sensor with electrical contact cleaner first before replacing it.

I would highly recommend purchasing this directly from Jeep for best accuracy/mating with your factory gauge. Many people including me have had mixed (poor) luck with aftermarket hardware.